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LONDON (AP) - Children who get several CT scans have a slightly higher chance of brain cancer and leukemia in later life, though the risk is still small and probably outweighed by the need to get the test, researchers reported.

The use of CT scans has risen rapidly since they were introduced 30 years ago. For children, they’re used to evaluate head, neck or spine injuries or neurological disorders.

International researchers studied nearly 180,000 patients under age 22 who had a CT scan in British hospitals between 1985 and 2002. They followed those patients until 2008. They found 74 of them were diagnosed with leukemia while 135 had brain tumors.

The scientists didn’t measure the number of scans, which were mostly of the head, but looked at data measuring radiation doses from the scans. That’s because the amount of radiation received by body parts such as the brain and bone marrow depends on the age and size of the patient.

The children who later developed leukemia or brain tumors were compared to a group of people who got a very low dose of radiation to the same parts of their bodies.

“CT scans are very useful, but they also have relatively high doses of radiation, when compared to X-rays,” said Mark Pearce of Newcastle University, the study’s lead author, at a press briefing Wednesday. He said CT scans were warranted in most situations but more needed to be done to reduce the amount of radiation.

Pearce and colleagues concluded the risk of brain tumors was tripled if children had two to three scans and the risk of leukemia was tripled with five to 10 scans. But he emphasized these were rare diseases and that the higher risk was still small. The risk of leukemia in children is about 1 in 2,000, so having several CT scans would bump that up to about 1 in 600.

“This (risk) is important, but the CT scan may be even more important,” said David Spiegelhalter, of the University of Cambridge. He was not connected to the research.

“A judgment has to be made,” he said in a statement.

The researchers noted that modern CT scanners give off about 80 percent less radiation than the older machines used in the study. Even at low doses, the radiation can damage genes that may increase the patient’s risk of developing cancer later.

The study was paid for by the U.S. National Cancer Institute and the U.K. Department of Health. It was published online Thursday in the journal Lancet.

In the U.K., laws already require radiation from medical scans be kept as low as possible. In the United States, the government is pushing manufacturers to design new scanners to minimize radiation exposure for the youngest patients. And it posted advice on the Internet urging parents to speak up when a doctor orders a scan _ to ask if it’s the best option or if there’s a radiation-free alternative _ and to track how many their child receives.

The American College of Radiology warned that fears of radiation should not prevent parents from getting necessary scans for their kids.

“If an imaging scan is warranted, the immediate benefits outweigh what is still a very small long-term risk,” Dr. Marta Schulman, chair of the group’s Radiology Pediatric Imaging Commission, said in a statement. “Parents should certainly discuss risk with their provider, but not refuse care that may save and extend their child’s life.”